1. Field of the Invention
This invention relates to an ion source for the production of an ion beam from gaseous species. More specifically, the ion source utilizes a plurality of electrically isolated electrodes exposed to a DC-type plasma, to sustain the plasma and to extract ions from the plasma in a way suitable for subsequent beam transport and focusing at variable energy.
2. Description of the Prior Art
Ion beams are commonly used for many purposes, among them material surface modification (implantation), doping of semiconductors, formation of compounds by epitaxy, surface analysis, sputter-etching and the like. For some applications, such as preparation of inorganic specimens for electron microscopy, it is desirable that the ion beams used be formed from noble gas species, such as argon, so as to minimize chemical interactions between the atoms of the beam and those of the specimen. Such noble-gas ion beams may still produce desired changes to a specimen by virtue of momentum transfer. It is generally further desirable to contain or otherwise control the beam diameter to reduce unwanted impingement of the beam on materials other than the specimen.
Ions that impinge on the surface of a specimen with sufficient momentum are capable of ejecting atoms of the specimen from its surface; this process is known as sputtering. The efficiency of the sputtering process depends on the incident ion's mass and kinetic energy, the specimen material, and the geometric parameters of the collision. Atoms that are sputtered from a specimen will eventually collide again with a surface, and could be adsorbed after the collision. It is possible that a sputtered atom could return to a site on the original specimen, near its original location before sputtering. This occurrence is known as “re-deposition.” For some applications, such as specimen preparation for electron microscopy, re-deposition is undesirable because it alters the original structure of the specimen. Hence, in some situations it is desirable to use an ion beam with a diameter no larger than the area intended to be sputtered, to minimize the likelihood that material from surrounding areas will be re-deposited onto the area of interest. There are many other well-known situations in which it is desirable to use an ion beam of small or limited diameter. In other applications, a wide beam is preferred for maximum coverage and milling of the specimen surface.
Various approaches have been tried for production of noble-gas ion beams capable of being focused into small diameter spots. These include gas-field ion sources, duo-plasmatrons, electron-bombardment ion sources, Penning ion sources, multi-cusp ion sources, electron-cyclotron resonance ion sources, and others as described in the literature. Each type has advantages and disadvantages in terms of output beam parameters, size, complexity, power requirements and cost, among other criteria.
For certain applications in specimen preparation for electron microscopy, the Penning-type ion source is a reasonable choice because of its small size and simplicity of construction. A class of commercial devices in this field, known as ion mills, typically provide ion beams with adjustable ion kinetic energy (known commonly as “energy”) in the range of approximately 500 eV (per ion) to 10,000 eV. Higher-energy beams provide faster milling, but may leave more residual surface damage, than lower-energy beams. So, it is useful to provide an adjustable energy beam, allowing the user to determine, on a case by case basis, the trade-off between processing speed and surface quality.
Penning-type ion sources use the principle of electromagnetic confinement to force electrons to make multiple passes through an ionizing region, known as a cavity, thereby increasing the likelihood of ionizing collisions between the energetic electrons and gas molecules in the cavity. More specifically, a magnetic field is used to constrain the motion of electrons to an axis along the length of the cavity, and electrostatic mirrors are placed at opposite ends of the cavity so that electrons generally cycle back and forth between the minors and along the magnetic axis. The electrostatic potential of the cavity is made higher than that of the minors by an electrode called the anode. So, electrons in the vicinity of the minors are attracted towards the cavity and acquire kinetic energy on their way into the cavity. The electrons oscillate between the mirrors and through the cavity until they collide with other particles, notably gas molecules introduced into the cavity. Some of these collisions result in the ionization of neutral gas molecules, resulting in additional free electrons and also ions. Ions formed in the cavity are attracted toward one mirror electrode or the other and on impact with a minor electrode, can liberate additional “secondary” electrons, which again are attracted to the anode. The result is a self-sustaining chain reaction, which produces a plasma in the cavity. By convention, because the minor electrodes function to introduce electrons, they are also identified as cathodes. In some embodiments one of the cathodes is heated to provide additional electrons to the plasma by thermionic emission, and sometimes the non-heated cathode is identified as an anti-cathode to emphasize the relative importance of the heated cathode for electron production.
The plasma is a gas with a high density of charged particles (ions and electrons), but approximately equal numbers of both polarities of charges, so that it has minimal net charge, i.e. minimal space charge. Because of the large number of free charged particles, the effective electrical conductivity within the plasma is high; i.e. the interior of the plasma is at an approximately uniform electrical potential. It is well known that this electrical potential, known as the plasma potential, is close to that of the anode. Between the interior of the plasma and any adjacent cathode, there is a region of relatively high electric field strength, known as a sheath.
Ions within the plasma are affected by diffusion processes, resulting in a net flux of ions exiting the boundaries of the plasma. This net flux out of the plasma offsets the ion generation process, resulting in an equilibrium concentration of ions within the plasma. Some of the diffusing ions collide with the anode and are neutralized, and others diffuse toward a sheath. When ions diffuse into a sheath they are accelerated quickly towards the adjacent cathode because of the strong electrostatic field in the sheath. Ions and electrons are quickly swept across a sheath by its high electric field; therefore, the density of charges within a sheath is relatively low.
In order to form a useful beam, ions must be extracted from the plasma in which they are produced. Generally, a hole is added to an electrode, so that ions that would normally impinge on the electrode in the area of the hole instead continue their motion through the hole and out of the source, where they can be used. Some embodiments provide a hole in the anode, while others provide a hole in a cathode. Generally, extraction through a cathode has advantages in terms of current density, as opposed to extraction through the anode. Ions extracted through a cathode exit the cathode with kinetic energy approximately equal to the difference in electrostatic potential between the plasma and the cathode, times the charge state of the ion. Adding a hole to a cathode for extraction reduces its ability to provide electrons to sustain the plasma. For this reason, and for purpose of specificity, in the instant disclosure, the cathode through which ions are extracted is identified as the anti-cathode, and the opposite minor electrode is identified as the cathode.
In general, the intended target of an ion beam is often located remotely from the ion source. This distance may range from a few millimeters to hundreds of millimeters or more. It is common in the field for the specimen, or workpiece, to be held at ground potential, along with the bulk of the vacuum chamber through which the ions pass. To prevent distortion of the beam in the region between the source and the specimen, it is usually desirable to minimize electrostatic fields in this region. This is often accomplished by adding a grounded electrode at the exit of the ion source.
FIG. 1 is a diagram of a typical prior art Penning-type ion source. Penning ion source 2 includes ring magnet 4, which creates an axial magnetic field in its interior region. Cathode 6 and anti-cathode 8 are made of metal with a high magnetic permeability and act as pole-pieces to shape the magnetic field. Cathode 6 and anti-cathode 8 are in electrical contact with magnet 4, which is also electrically conductive. Cathode 6 is attached to insulating base 5, which is mounted on vacuum flange 7, which in turn is mounted to a vacuum chamber (not shown). The vacuum chamber is connected to earth potential and so flange 7 is also at earth potential. Anode 10 is ring-shaped and supported by circumferentially disposed insulator(s) 12, thus anode 10 is electrically isolated from the other system components. Anode 10 is connected to a power supply (not shown), biased positive at nominal ion beam potential with respect to the vacuum chamber, through electrical feed-through 14, which passes through cathode passage 13 and through insulating base 5. Ion beam potential is defined as the desired kinetic energy per ion outside the source, divided by its nominal charge state. Cathode 6 is connected to a second power supply (not shown) through electrical feed-through 18, which biases cathode 6, magnet 4 and anti-cathode 8 negatively with respect to anode 10. Gas is supplied through inlet tube 22, flows through the ion source 2, and is pumped out of the source into the vacuum chamber (not shown) through aperture 24 in flange 7, thereby maintaining a gas pressure inside anode 10 that is favorable for sustaining a Penning discharge. A plasma is formed in the interior area of anode 10; the plasma potential is approximately the same as that of anode 10. Some ions from the plasma are directed towards anti-cathode 8 due to the electric field within the plasma sheath in the vicinity of anti-cathode 8, and some of these ions drift through anti-cathode passage 9 and are extracted to the right. Ions attain their final energy once they pass through aperture 24, which is at ground potential.
One of skill in the art will appreciate the fact that the beam divergence in the vicinity of the anti-cathode (i.e., in the area of initial extraction from the plasma), is affected by the shape of the plasma boundary and the shape of the anti-cathode electrode. The shape of the plasma boundary is affected by many factors, among them the pressure of gas inside the anode volume, the discharge power, the potential difference across the plasma sheath, and the shapes of the electrodes. Further, for a given geometry and gas pressure, a certain minimum potential difference is required between the anode and mirror electrodes to sustain a plasma discharge. Typically this minimum potential difference is on the order of 700V. In the prior art Penning ion source 2, this potential difference appears across the plasma sheath in the vicinity of anti-cathode 8, thereby affecting the divergence of the extracted beam. Therefore, the requirements for generating ions through plasma discharge impose restrictions on the beam divergence at the point of extraction.
A divergent beam can be made more parallel by using an electrostatic lens. In Penning ion source 2, the space between anti-cathode 8 and aperture 24, in general, constitutes a lens, since under most operating conditions there is a potential difference between anti-cathode 8 and aperture 24. The strength of this lens varies in proportion to the potential difference between anti-cathode 8 and aperture 24, so that as the anode potential is raised to high values, e.g., 5000V, the lens becomes fairly strong and corrects for the high initial divergence of the beam. However, at lower anode potentials, the strength of this lens becomes progressively weaker, so that the initial beam divergence cannot be fully corrected by the lens-effect between anti-cathode 8 and aperture 24, and the spot size on the specimen increases.
It will be apparent to those skilled in the art that one could place an additional lens or lenses between anti-cathode 8 and aperture 24, or to the right of aperture 24. This is not desirable as it adds to the mechanical size and complexity of the system, requires a separate power supply, and introduces new aberration factors into the beam path. Further, its power supply may be required to generate impractically high voltages, perhaps substantially greater than that of the anode power supply, to provide sufficient focusing.
For the reasons articulated above, the production of a parallel or nearly parallel ion beam over a wide energy range with the prior art Penning ion source 2 is not practical. For ion milling, this means that a Penning ion source, which is optimized for spot size at one energy level, will generally suffer increased spot size when operating at other energy levels. Hence, re-deposition of sputtered material onto an area of interest is likely when operating at energies away from the design point.
As will be apparent to those skilled in the art, the use of lower energy ions would minimize specimen damage. However, the ability to create a small, focused beam at low energy has not been resolved.
What is lacking in the art, therefore, is an ion source with the ability to thin a specimen to electron transparency with a low energy ion beam having a relatively small beam diameter.